The great diversity of immunoglobulin (Ig) antibody specificities is largely encoded in the immunoglobulin variable region genes (V genes). The phenotypic expression of these V genes in vivo during the course of an antibody response is under the control of distinct subsets of T lymphocytes. The purpose of the experiments in this proposal is to define the specificity and functional importance of T cells involved in inducing the production of antibody bearing a particular idiotypic determinant as a marker for V gene expression. I have recently described two antigen specific helper T cell (Th) populations required for dominant production of antibody bearing a particular idiotype. In these experiments, the precision of recognition for the foreign antigen, for self major histocompatibility complex determinants and for self idiotype will be independently determined for these two Th cell sets. The T cell signals received by individual B cells from each Th cell set will also be analysed using limiting dilution in vitro assays. One major purpose of the proposed experiments will be to better define the biological significance of a specialized set of helper T cells recognizing idiotypic precursor B cells and amplifying their activity. It is my hypothesis that these Th cells are required for the rapid and early activation of that set of B cells bearing the most frequently encountered idiotypes, even when this amplified response is of relatively low affinity. This will be tested by determining the kinetics of the response in the presence or absence of this Th cell set.